(1) Field of the Invention
This invention relates to a method of making and the resultant structure for a DRAM (Dynamic Random Access Memory) cell, and more particularly to a method of making, and the resultant structure of, a DRAM stack capacitor with a adder storage node.
(2) Description of the Related Art
A typical DRAM cell consists of a single transistor and a storage capacitor. Digital information is stored in the capacitor and accessed through the transistor, by way of addressing the desired memory cell, which is connected with other such cells through an array of bit lines and word lines. In order to construct high density DRAMs in a reasonably sized chip area, both the transistor and capacitor elements must occupy less lateral space in each memory cell. As DRAMs are scaled down in dimensions, there is a continuous challenge to maintain a sufficiently high stored charge per capacitor unit area. Efforts to increase capacitance without increasing the planar area of the capacitor have been concentrated on building three dimensional capacitor structures, which increase the capacitor surface area. Thus cell structures have to change from the conventional planar-type capacitors to either trench capacitors or stack capacitors, in particular at densities above 4 Mbit.
When the stacked capacitor approach is used, in order to maintain sufficient capacitance the storage node must have a large surface area, and consequently must be formed significantly above the surface of the substrate in which the DRAM cell is formed, thus leading to topological problems in the formation of subsequent layers.
Workers in the art are aware of these problems, and have attempted to resolve them. For example, in U.S. Pat. No. 5,183,772 (Jin et al) a method is shown for forming a stack capacitor as part of a DRAM cell, in which the capacitor is formed with a saddle shape to reduce later topological problems. However, the saddle shape is an asymmetrical structure with a substantially vertical side slope 50 as can be seen in the cross-sectional view in FIG. 13. The steep slope contributes to continued difficulties with the topology of subsequent layers.